Glass optical fibers are widely used in the communications field because of their ability to carry large amounts of information over long distances. In order to protect the fibers from physical damage and from deterioration due to environmental agents such as water, it is conventional to apply at least one protective coating immediately after spinning the molten glass. Generally, two coatings are applied, a soft primary coating of low modulus and low glass transition temperature (Tg) directly on the glass surface, and a harder (higher Tg), higher modulus secondary coating, on the primary coating. The individual fibers generally are combined in larger structures such as cables. Cables may comprise individual fibers, or fiber ribbon structures. The optical fiber ribbon generally is made from 2, 4, 6, 8 or 12 optical fibers, in general arranged in a plane, and bonded together with a so-called matrix material. Several ribbons can be bundled together using bundling materials. Further, individual fibers often are provided with a coloring or ink layer to be able to identify individual fibers.
Resins that cure on exposure to radiation such as ultraviolet radiation are favored in the industry, due to their fast cure, enabling the coated fiber to be produced at high speed. In many of these radiation curable resin compositions, use is made of urethane oligomers having reactive termini (such as f.i. an acrylate or methacrylate functionality) and a polymer backbone, which compositions generally further comprise reactive diluents, photoinitiators to render the compositions radiation-curable, and suitable additives.
There are several properties which are commonly required for all the different coatings. Some of these requirements are low water absorption, low extractables, maintenance of the desirable levels of properties such as modulus, elongation, Tg, and adhesion under aggresive aging conditions (over relatively long periods of time) including high temperatures and/or high humidities, immersion in water and chemical resistance.
As mentioned above, urethane acrylate oligomers are well-known for use in radiation curable coating compositions for optical fibers. As polymer backbone for the urethane oligomer, many materials have been used, such as polyether backbones. Urethane acrylate oligomers are derived from reactants including a hydroxy-functional prepolymer (polyol), an organic diisocyanate and an ethylenically unsaturated monomer containing one or more hydroxyl groups. Polyols that often have been used are f.i. hydrocarbon polyols, polyether polyols such as poly(ethylene)glycol, poly(propylene)glycol and poly(tetrahydrofuran)diols, polyester polyols, polycarbonate polyols and polysiloxane polyols. Polyester polyols are particularly attractive because of their commercial availability, oxidative stability and versatility to tailor the characteristics of the coating by tailoring the backbone. One of the problems encountered in using most polyesters is their hydrolytic instability and oil sensitivity.
In this respect, a continuous search is going on in the field of optical fiber coatings for alternative coatings with suitable properties. In WO 98/56846, it is suggested to improve the resistance to hydrolysis by using (a urethane oligomer with) a polyester backbone consisting mainly of polymerized diol and diacid and/or hydroxy acid components wherein the diol component has at least one carbon at the β-position with respect to the hydroxyl groups which is substituted with a group containing at least one carbon atom. The disadvantage of the polyesters described in WO 98/56846 is that they have a relatively high viscosity and hence, a higher concentration of diluents is needed, thus limiting the selection of properties.
In JP-A-2216947 a urethane acrylate oligomer is suggested in which the backbone can be an alkyd polyester that can be obtained by reacting a fatty acid such as soy oil, linseed oil, safflower oil, palm oil, dehydrated ricinus oil, dehydrocastor oil, or tung oil with a polyacid and a polyol. The radiation-curable optical fiber inner primary composition comprising the urethane acrylate oligomer described in JP-A-2216947 is solvent-based, which clearly is a drawback. Moreover, the oils on which alkyds in JP-A-2216947 are based all contain considerable unsaturation.
Among many other efforts, EP-A1-539030 suggests to use polyester polyols wherein the acid-component and/or the hydroxyl component residues of the polyester polyol are partially comprised of dimer acids. The dimer acids suggested in EP-A1-539030 are usually unsaturated, but can also be hydrogenated to remove a substantial proportion of the unreacted double bonds. However, dimer acids have the disadvantage of swelling in hydrocarbon oil, which is representative of cabling gel filling type compounds. U.S. Pat. No. 4,629,287, U.S. Pat. No. 4,609,718, and U.S. Pat. No. 4,608,409 all mention the use of dimer fatty acids which are dicarboxylic acids formed by dimerizing fatty acids which usually contain 18 carbon atoms, and thus providing a 36 carbon atom dicarboxylic acid.
So far, very few alternatives for fiber optic coatings with appropriate coating characteristics, such as a combination of a low water sensitivity and a low oil sensitivity, have been found.